Specify an observer by coordinate

For coordinate frames that include the observer location as part of the definition (e.g., Helioprojective), there are two main ways to specify the observer location (the observer frame attribute). If it makes sense to designate a planetary body as the observer, one can specify the observer to be that body using a string (e.g., observer='earth'), and the observer will be understood to be situated at the center of that body at the time of the coordinate (the obstime frame attribute). However, if the precise location on a planetary body is important, or if the observer is elsewhere in the space (e.g., a spacecraft), one can specify that observer location using a SkyCoord object.

>>> from astropy.coordinates import SkyCoord, CartesianRepresentation
>>> from sunpy.coordinates import frames
>>> import astropy.units as u

The SkyCoord for the observer location can come from a variety of sources. If you are working with a Map with an observer location stored in the metadata, you can retrieve the observer location using the observer_coordinate property. If the observer is a major spacecraft, you might be able to obtain its location using get_horizons_coord(). Here, we will manually specify an observer location in Stonyhurst heliographic coordinates (HeliographicStonyhurst):

>>> observer_coord = SkyCoord(70*u.deg, -30*u.deg, 1*u.AU, frame=frames.HeliographicStonyhurst)
>>> observer_coord
<SkyCoord (HeliographicStonyhurst: obstime=None, rsun=695700.0 km): (lon, lat, radius) in (deg, deg, AU)
    (70., -30., 1.)>

Now we can create a coordinate in helioprojective coordinates (Helioprojective) using the observer location defined above:

>>> coord = SkyCoord(100*u.arcsec, 200*u.arcsec, frame=frames.Helioprojective, observer=observer_coord)
>>> coord
<SkyCoord (Helioprojective: obstime=None, rsun=695700.0 km, observer=<HeliographicStonyhurst Coordinate (obstime=None, rsun=695700.0 km): (lon, lat, radius) in (deg, deg, AU)
    (70., -30., 1.)>): (Tx, Ty) in arcsec
    (100., 200.)>